In the prior art, the forged and NDT (Non-Destructive Testing) tested discs are machined for weld seam preparation. Afterwards the discs are stapled on top of each other and their run-out relative to each other and as a whole is checked and if necessary adjusted. Now, with the discs in vertical position the root of the weld is melted without weld filler using Tungsten Inert Gas welding (TIG). After this the weld height in radial direction is increased to allow tilting of the rotor in horizontal position using very narrow gap TIG with parent metal weld filler (reinforcement layers to allow tilting of the rotor). After tilting the rotor in horizontal position, the welding is finalized, filling the weld using Submerged Arc Welding (SAW). Finally the welds of the rotor are NDT checked using US (ultrasonic) testing.
The prior art welding of rotors for power generation equipment is described in more details with EP 0 665 079 A1 and EP 2 215 329 A1.
Documents EP 0 665 079 A1 and EP 2 215 329 A1 (resp. WO 2009/065739 A1) describe the principles for welding rotors for power generation (gas turbines, steam turbines, generators), which comprise a plurality of rotor discs arranged along a rotor axis, thereby using TIG welding, especially very narrow gap TIG for the root area of the welding joint, and afterwards Submerged Arc Welding (SAW) to fill the weld seam.
FIG. 1 (similar to EP 0 665 079 A1) shows a prior art welding joint with a welding seam 13 between two adjacent rotor discs 11 and 12 of a welded rotor 10. Before welding the two discs 11 and 12 abut with a centering step joint 20 of height h, thereby creating an extreme narrow gap 19, which is defined by extreme narrow gap component faces 23 and 24 of the discs 11 and 12, and a narrow gap 14, which is defined by narrow gap component faces 15 and 16 of the discs 11 and 12.
The extreme narrow gap 19 is filled with TIG welding seams 25, each welding seam extending over the full extreme narrow gap width b. The narrow gap 14 is filled with SAW welding seams 17, 18, which are smaller than the narrow gap width a and are alternating butt against the opposing narrow gap component faces 15 and 16, and overlap in the middle of the narrow gap 14. During TIG welding a fusing area 22 at the root of the welding joint is fused. Furthermore, a relieving ridge 21 with a relieving ridge width c and height d and relieving ridge angle a is provided at the root of the welding joint.
The extreme narrow gap TIG weld (welding seams 25) are welded typically in a vertical stack of rotor discs (see for example FIG. 2 of WO 2009/065739 A1) and the subsequently filled SAW weld (SAW welding seams 17, 18) is filled in horizontal position (see for example FIG. 3 of WO 2009/065739 A1). The TIG-SAW transition is shown with a U type weld joint preparation for the SAW area.
The typical dimensions of such a prior art welding joint are:
Typical width of TIG extreme narrow gap: b=10 mm.
Typical width of SAW narrow gap: a=17 mm.
Document EP 0 665 079 A1 discloses a method, which concerns joining of metal components by arc fusion welding which produces a low-volume seam made up of weld beads applied alternating to the component flanks according to the process specified in DE 26 33 829 A1. In addition to a normal gap, the components are provided with an adjoining end gap and a centering insert. The components are positioned with the aid of this insert, a relief swelling is formed in the region of the seam bottom, and a required through-fusion position is produced by the welding process. Subsequently the component flanks are joined by fully automatic TIG welding or MPG welding produces multi-layered weld beads whose width covers the entire width b of the end gap.
Document EP 2 215 329 A1 or WO 2009/065739 A1, respectively, relates to a method for producing a rotor made by welding together disc and/or drum-shaped elements, particularly discs, wherein said discs are assembled in sequence along a longitudinal axis by means of a device and welded in a two-stage welding process. The discs are stacked axially in the vertical direction during assembly. A first welding process takes place in a vertical orientation of the stacked discs, followed by a second welding process in a horizontal orientation of the stacked discs.
Nowadays, in the field of welded rotors for turbomachines, there are other powerful weld procedures available, which could replace Tungsten Inert Gas (TIG) welding for the melting of the root and the filling of the reinforcement layers with high quality and reduced cost.
Metal Active Gas (MAG) fully automatic process is a powerful process to replace the Submerged Arc Welding (SAW) process for filling up the weld after root melting and welding of the reinforcement layer. MAG welding can also be applied in vertical position if necessary. In this case no reinforcement layers to allow tilting before SAW welding would be necessary, but immediately after melting of the root area, the weld can be filled in vertical position by MAG.